In the production step of printed circuit boards, flexible printed circuit boards, multilayer printed circuit boards, and the like, a mold releasing film is used at the time when a copper-clad laminate or a copper foil is hot-pressed through a prepreg or a heat-resistant film. Moreover, in the production step of flexible printed circuit boards, a method of inserting a mold releasing film is widely performed for the purpose of preventing adhesion of a cover-lay film to a pressing hot plate at the time when the cover-lay film is subjected to hot press bonding using a thermosetting adhesive to a flexible printed circuit board main body where electric circuits are formed, or for the purpose of preventing adhesion of printed circuit boards to each other or protecting the printed circuit board products at the time when a plurality of monolayer or multilayer printed circuit boards are produced simultaneously.
Recently, in view of increasing social requests for environmental problems and safety, an anti-moisture absorbing property, rigidity and contamination resistance have been required for the mold releasing films in addition to the functions such as heat resistance for enduring hot pressing and mold-releasing property from printed circuit boards (inclusive of PI, epoxy resins, epoxy adhesives, copper foil, etc.) and hot pressing plates. The absorption of moisture may cause blisters by generation of water vapor during the hot pressing. When rigidity is low, the film tends to be wrinkled and suffers from a problem of poor workability. With regard to the contamination resistance, if organic or inorganic components bled out of the mold releasing film are transferred to the surface of the printed circuit board, plating in a subsequent step does not proceed satisfactorily, a washing step is required, or sometimes, a significant problem arises in view of product safety for such products, which should not contain impurities, such as hard disk drives.
However, polymethylpentene films, silicone-coated polyester films, fluorine-based films, and the like which are hitherto used as mold releasing films do not sufficiently satisfy the above performances required for a mold releasing film. Namely, when polymethylpentene films are hot pressed and cooled, a printed circuit board may be sometimes wrinkled by thermal shrinkage. Or, some measure such as a change of the shape of the printed circuit board product must be made so as to avoid wrinkling and hence the freedom of the shape and design is sometimes restricted. Furthermore, in view of the presence of slight transferred matter onto the printed circuit board, application of the printed circuit board product is sometimes restricted and thus the film cannot be considered as being sufficient. The silicone-coated polyester films have an insufficient heat resistance and also there exists a possibility that the quality of products such as printed circuit boards may be impaired by migration of silicone. The fluorine-based films are excellent in heat resistance and mold releasing property but are expensive and have a problem of poor workability owing to low rigidity. Although a crosslinked resin-type mold releasing film is proposed, its rigidity and anti-moisture absorbing property are not sufficient and it involves a problem that a complex process, i.e., a crosslinking step is required. See Patent Documents 1 to 4.    Patent Document 1: U.S. Pat. No. 4,777,201    Patent Document 2: Japanese Patent No. 2790330    Patent Document 3: JP 2003-053896 A    Patent Document 4: JP 2003-012829 A
Moreover, a film mainly containing syndiotactic polystyrene has been proposed as a mold releasing film but the wrinkling problem is not sufficiently solved (see Patent Documents 5 to 7).    Patent Document 5: JP 11-349703 A    Patent Document 6: JP 2000-038461 A    Patent Document 7: JP 2001-246635 A
Furthermore, an alicyclic polyolefin film has been proposed as a mold releasing film but, from the viewpoint of workability (rigidity) and cost, it is not satisfactory (see Patent Document 8).    Patent Document 8: JP 2001-233968 A
In addition, a film comprising a polyamide resin has been proposed as a mold releasing film but, from the viewpoint of the mold releasing property with an adhesive, it is not satisfactory (see Patent Document 9).    Patent Document 9: JP 6-316032 A
Moreover, a polyether aromatic ketone resin film has been proposed as a mold releasing film but it is not satisfactory since the resin is still a crystalline resin and hence requires a step of heat treatment for covering the problem of thermal shrinkage and from the viewpoint of cost (see Patent Document 10).    Patent Document 10: JP 2003-236870 A
As above, in the conventional technologies, there has been no monolayer releasing film for printed circuit board production that substantially contains PPE as a main component. Since most conventional mold releasing films are constituted by crystalline resins, the temperature for curing an adhesive (mainly from about 130 to 180° C.) exists in the range between their melting point and their glass transition temperature in many cases. Therefore, in the heating and cooling steps of hot pressing, it is difficult to avoid the wrinkle problem of the mold releasing films themselves and of printed circuit board products caused by thermal shrinkage of the mold releasing films.
On the other hand, in addition to the function of the mold releasing property against base films of flexible printed circuit boards represented by polyimide or hot pressing plates, a shape-following property, less overflow of adhesive, adhesion between multilayer films, and slipping property between films are required. The shape-following property means a cushion property of following differences in levels of the polyimide film and copper foil to be adhered thereto and mitigating the impact upon application of pressure during hot pressing. The overflow of an adhesive means the extent of overflow of the adhesive onto an electrode part at a hole where the copper foil is exposed, which is called land, in a circuit placed between base films such as polyimide or cover-lay films. A smaller amount is better. This is because greater overflow may sometimes result in connection failure when connection of a circuit to the electrode is attempted by soldering or the like. Moreover, the reasons why adhesion between multilayer films is required are as follows: A wrinkle problem arises in products, lamination of a third resin layer is needed for adhesion between a surface-layer resin and an intermediate-layer resin and thus the whole film becomes thick, and the structure of a laminate film and a film producing step become complicated. The generation of wrinkles in products may be attributed to generation of an air layer between the layers when the adhesion between the surface-layer resin and the intermediate-layer resin is weak. The reason why the slipping property is required between the films is that, when the slipping property is poor, the films may stick to each other, which causes a decrease of working efficiency.
Although mold releasing films exhibiting a high shape-following property which comprises a multilayer film containing an intervening resin having a low heat resistance as an intermediate layer have been proposed, the conventional technologies cited below are not satisfactory in view of achieving high levels of the shape-following property, less overflow of an adhesive, the interlayer adhesion between the multilayer films, and the slipping property of the film simultaneously (see Patent Documents 11 to 16).    Patent Document 11: JP 2000-263724 A    Patent Document 12: JP 2000-272055 A    Patent Document 13: JP 2003-001772 A    Patent Document 14: JP 2003-246019 A    Patent Document 15: JP 2003-276139 A    Patent Document 16: JP 2001-310422 A
As above, in the conventional technologies, there has been no multilayer mold releasing films for printed circuit board production that comprises as the outermost layer, a resin layer substantially containing PPE as a main component. Also, they have not been satisfactory in slipping property, i.e., workability, or less sticking to printed circuit board products.